With the present-day surge in demand for communications, long-distance optical submarine cable systems are now being actively constructed. A transoceanic long-distance optical submarine cable system is usually configured with end-office equipment disposed in a cable landing station at either end of the system and submarine repeaters located at about 50 km repeater spacing.
For efficient transmission of high-volume information over such an optical submarine cable, there is available a wavelength division multiplex (WDM) optical transmission technique. For example, 100-wavelength multiplexing of a signal of a bit rate 10 Gb/s per wavelength will permit transmission of a 1-Tb/s volume of information.
The transmission capacity could be increased by increasing the number of WDM wavelengths. For example, by setting the number of wavelengths at 200, it is mathematically possible to increase the overall transmission capacity of the cable system up to 2 Tb/s.
However, an increase in the number of wavelengths requires doubling the bandwidth of each repeater. With a 0.4 nm wavelength spacing, a 40 nm bandwidth is required for 100 wavelengths; this bandwidth requirement could be met by the provision of an erbium-doped fiber amplifier (EDFA) equipped with a precisely designed gain equalizer, but no means has been found so far for implementing a 80 nm bandwidth necessary for 200 wavelengths.
Another possible solution to this problem is to raise the bit rate per wavelength. For example, if the bit rate per wavelength is increased to 40 Gb/s, the required number of wavelengths for achieving the overall transmission capacity of 2 Tb/s is merely 50 wavelengths, and even if the wavelength spacing is 0.8 nm, a 40 nm bandwidth is enough for the repeater.
To raise the bit rate per wavelength as mentioned above is effective in increasing the overall transmission capacity of the cable system, but it gives rise to a new problem for long-distance transmission.
In general, it is known in the art that the reach over a dispersion transmission line as of an optical fiber decreases in inverse proportion to the square of the bit rate. This means that a 8.000 km distance of transmission at 10 Gb/s goes down to 1/16, that is, 500 km at 40 Gb/s.
The solution of this problem calls for an extremely precise dispersion management and dynamic dispersion compensation control in the end-office equipment, inevitably leasing to an increase in the cost of the entire system. It is also said that transmission is difficult to conduct at the cube of the bit rate owing to a nonlinear phenomenon in the optical fiber (self phase modulation, for instance) as well as the influence of dispersion.
A promising solution to this problem by increasing the transmission rate per unit time without raising the bit rate is a multi-level modulation technique conventionally used in the field of radio transmission. The multi-level modulation is referred to also as an M-ary modulation; according to this scheme, the transmitting side transmits binary information after encoding it into an M-ary signal, and the receiving side decodes it into the binary information.
FIG. 1 is a block diagram showing conventional optical transmission equipment set forth, for example, in “Journal of Lightwave Technology, pp. 2235–2248, vol. 17, no. 11, November 1999.” In FIG. 1, reference numeral 1 denotes a binary source for generating binary information; 2 denotes a multi-level encoder for encoding the binary information from the binary source 1 into a multi-level signal; 3 denotes an optical modulator for optical modulation of the multilevel signal encoded by the multi-level encoder circuit 2; 4 denotes an optical fiber transmission line; 5a to 5d denote optical amplifiers; 6 denotes an optical receiver for receiving the multi-level signal sent over the optical fiber transmission line 4; 7 denotes a multi-level decoder for decoding the multi-level signal received by the optical receiver 6 into a multi-level signal to reconstruct the binary information; and 8 denotes the reconstructed binary information.
Next, the operation of the illustrated prior art example will be described below.
Let it assumed here that the binary source 1 generates binary information of a 40 Gb/s bit rate.
On receiving the 40 G/s binary information from the binary source 1, the multi-level encoder encodes the 40 Gb/s binary information to perform a quarternarary amplitude modulation (QAM), and outputs a 20 Gb/s QAM signal.
Upon receiving the 20 Gb/s QAM signal from the multi-level encoding circuit 2, the optical modulator 3 modulates the QAM signal into a 20 Gb/s 4-ary or 4-level NRZ (Non-Return-to-Zero) optical signal, and outputs it onto the optical fiber transmission line 4.
Thereafter, the optical signal is transmitted over the optical fiber transmission line 4 to the optical receiver 6 while being amplified by the optical amplifiers 5a to 5d. 
On receiving the 20 Gb/s 4-ary NRZ optical signal, the optical receiver 6 converts it to an electrical 4-ary NRZ signal, and provides it to the multi-level decoder 7.
On receiving the electrical 4-ary NRZ signal from the optical receiver circuit 6, the multi-level decoder 7 decodes the 4-ary NRZ signal to reconstruct the binary information 8.
The above technique is disclosed in “Multi-Level Dispersion Supported Transmission at 20 Gbit/s over 46 km Installed Standard Singlemode Fibre” published by Wedding, et al. in ECO '96, pp. 19–94, 1996, Oslo. Further, the above technique has the advantage that the use of 20 Gb/s optical transmission equipment enables a virtual 40 Gb/s transmission as experimentally verified in G. Veith, “European 40 Gbit/s Field Tests,” ECO '99, p. 11–82–83, 26–30, Sep. 1999, Nice. The experiments by G. Veith demonstrate the virtual 40 Gb/s transmission.
With the conventional optical transmission equipment of the above configuration, the optical signal transmission has been proved possible over an about 111 km reach but not proved for a reach in the 8.000 meter class; hence, the prior art equipment has the problem that its application to a transoceanic optical submarine cable system is difficult.
The present invention is intended to solve the above problem, and has for its object to provide optical transmission equipment and method that permit implementation of transoceanic long-distance transmissions.